1. Field of the Invention
The present invention relates to a liquid crystal compound containing a tolan derivative and a liquid crystal device (LCD) using the compound, and more particularly to a homeotropic structure type liquid crystal display device adopting a DAP mode.
2. Description of the Related Art
It is a general tendency that so far manufactured liquid crystal display devices adopt a TN mode or an STN mode which utilizes a nematic liquid crystal having a positive dielectric constant anisotropy. However, while these operation modes rely on twisted liquid crystal molecules, the twisted liquid crystal molecules appear to be different if the display device is viewed from different directions. As a result, the display device has a disadvantage that its contrast varies depending on a visual angle.
As a solution for eliminating the above disadvantage, a display device of a DAP mode utilizing a nematic liquid crystal with a negative dielectric constant anisotropy has been under consideration. In this mode, liquid crystal molecules are arranged substantially perpendicular to glass substrates in an initial arrangement of liquid crystal (when applied with a voltage equal to or less than a threshold value). Thus, the DAP mode has an advantage that the arrangement of liquid crystal molecules is the same regardless of visual angle and that the contrast has a low visual angle dependency. Under such background, studies on display devices of the DAP mode have been under progress.
To utilize a DAP mode liquid crystal for a display device, it is required that an abrupt threshold characteristic or abruptness .gamma.(V90/V10) is small, where V90 represents a voltage at which 90% of transmissivity is obtained, while V10 represents a voltage at which 10% of transmissivity is obtained.
Also, to realize a projection type liquid crystal display using this liquid crystal, a wide range of liquid crystal phase temperatures is required.
A transmitted light intensity I in the DAP mode is expressed by: EQU I=I.sub.0 Sin.sup.2 2.theta.Sin.sup.2 (.pi.R/.lambda.)
where I.sub.0 is an incident light intensity, .theta. is an angle formed by an incident polarized light direction and a vibration direction of ordinary light in a liquid crystal cell, and .lambda. is the wavelength of incident light. Further, R represents the amount of retardation of the liquid crystal cell, which depends on a cell thickness d, birefringence .increment.n of a liquid crystal and an inclined angle .phi.(V) of the major axis of liquid molecules, as expressed by: EQU R=d.increment.n Sin.sup.2.phi. (V)
It can be seen from the above equation that, to improve the abruptness .gamma., it is necessary to increase birefringence .increment.n of liquid crystal as well as employ cells having a larger thickness d.
It is an object of the present invention to provide a liquid crystal material for use in the DAP mode, which has a wide liquid crystal phase temperature range even applicable to a projection type liquid crystal display, increased birefringence and improved abruptness .gamma..